What next glass?

Have SWMBO and personal sense of thrift. Am now needing to increase my lens total from two to three; I have a DA* 16-50 and a used Vivitar I bought that is the coolest little devil at moon shots (and I have taken hundreds) and gave lots of really good nights. All this use with a K-7 beauty. But I need, in a Maslovian sense, the DA* 50-135. At $800 on Adorama, should I buy it used for $700 - 750. Do I get want the security of a warrenty so I should pay the extra for new? Or, is there other glass that you would add for most of my pictures; nature and geography, other outdoor, and a small mix of indoor pics and moon shots/star trails. I live and love in the Southwest Desert and appreciate the ruggedness that weather resistent lens bodies provide. And don't forget about the SWMBO

Have SWMBO and personal sense of thrift. Am now needing to increase my lens total from two to three; I have a DA* 16-50 and a used Vivitar I bought that is the coolest little devil at moon shots (and I have taken hundreds) and gave lots of really good nights. All this use with a K-7 beauty. But I need, in a Maslovian sense, the DA* 50-135. At $800 on Adorama, should I buy it used for $700 - 750. Do I get want the security of a warrenty so I should pay the extra for new? Or, is there other glass that you would add for most of my pictures; nature and geography, other outdoor, and a small mix of indoor pics and moon shots/star trails. I live and love in the Southwest Desert and appreciate the ruggedness that weather resistent lens bodies provide. And don't forget about the SWMBO

I own both the 16-50 and the 50-135. Bought the 16-50 used and the 50-135 new. I would definitely buy the new one for that little price difference. Just the piece of mind is worth that much. If you do nature, I might consider the DA*300mm or something like a Sigma 50-500mm. I have the Sigma and really have taken some great pictures with it but must warn you that a Monopod is required at 500mm unless you have nerves of steel. I would like to get the 300mm though for the IQ. Just wish Pentax would make a TC for it to increase its reach for wildlife. If they did, I could probably do without the Sigma.
My 50-135 does not get used much. Mainly at family events like weddings or graduations. Have thought of selling it at times but can't bring myself to part with it. It is probably one of the best if not the best DA* lenses Pentax makes.

Gary, I don't see myself doing a lot of wildlife that I can't do with the Vivitar. It is a 160-600mm, but is so big that you can't really use it well without a tripod. But I love the night shots of the moon. And it was used for $350. I easily have received $365 dollars worth of fun. But I have found that the 50 mm just isn't quite long enough for a lot of my shots, especially outdoors, and I end of cropping them post processing. I like the DA*16-50, and would probably love the 50-135. But you would buy it new at that pricepoint. Thanks for the advice. Is there a formula for deteriming new or used?

I would definitely buy it new at $800. I gave $550 for the 16-50 used. New at B&H it is $749. I gambled and figured $200 for a possible SDM repair down the road would be worth the gamble. $800 at Adorama for the 50-135 looks like a pretty good deal as the used ones generally sell for $7-$750.